Linux kernel built with the IPv6 protocol(CONFIG_IPv6) along with the IPv6 source address based routing
support(CONFIG_IPV6_SUBTREE) is vulnerable to a NULL pointer dereference flaw. It could occur while doing an
ioctl(SIOCADDRT) call on an IPv6 socket. User would need to have CAP_NET_ADMIN privileges to perform such a call.

When the kernel is compiled with CONFIG_IPV6_SUBTREES, and we return
with an error in fn = fib6_add_1(), then error codes are encoded into
the return pointer e.g. ERR_PTR(-ENOENT). In such an error case, we
write the error code into err and jump to out, hence enter the if(err)
condition. Now, if CONFIG_IPV6_SUBTREES is enabled, we check for:

Since pn is NULL and fn is f.e. ERR_PTR(-ENOENT), then pn != fn
evaluates to true and causes a NULL-pointer dereference on further
checks on pn. Fix it, by setting both NULL in error case, so that
pn != fn already evaluates to false and no further dereference
takes place.

This was first correctly implemented in 4a287eba2 ("IPv6 routing,
NLM_F_* flag support: REPLACE and EXCL flags support, warn about
missing CREATE flag"), but the bug got later on introduced by
188c517a0 ("ipv6: return errno pointers consistently for fib6_add_1()").

A user/program with CAP_NET_ADMIN privileges could use this flaw to crash a
system resulting in DoS.